Various electrical water heating units combining domestic water heating and space heating are known; one such system is described in U.S. Pat. No. 3,502,843 issued Mar. 24, 1970 to Stryer. This unit consists of two boilers, one nested within the other, which provide hot water for heating a space and domestic hot water for washing and other purposes. The interchange of heat is effected between the boilers which, although nested within the one another, do not allow hot water from one boiler to exchange heat to the water of the other boiler by circulating through it. There is, therefore, no constant transfer of heat from one system to the other to mimimize the use of heating elements.